It has become desirable to employ xerographic nonimpact printers for text and graphics. An electrostatic charge is developed on the photoreceptive surface of a moving drum or belt and selected areas of the surface are discharged by exposure to a light. A printing toner is applied to the drum and adheres to the areas having an electrostatic charge and does not adhere to the discharged areas. The toner is then transferred to a sheet of plain paper and is heat fused to the paper. By controlling the areas illuminated and the areas not illuminated, characters, lines and other images may be produced on the paper.
One type of non-impact printer employs an array of light-emitting diodes (LEDs) for exposing the photoreceptive surface. A row or two closely spaced rows of minute LEDs are positioned near a lens so that their images are arrayed across the surface to be illuminated. The LEDs along the row are selectively activated to either emit light or not as the surface moves past, thereby exposing or not exposing the photoreceptive surface in a pattern corresponding to the LEDs activated.
To obtain good resolution and image quality in such a printer, the physical dimensions of the LEDs must be quite small and very tight position tolerances must be maintained. Dimensional tolerances are often no more than a few tens of micrometers.
The light emitting diodes for such a printer are formed on gallium arsenide chips or dice by conventional techniques. Arrays of LEDs are formed on a large wafer which is then carefully cut into individual dice, each elongated die having a row of LEDs along its length. An exemplary die about eight millimeters long may have 96 LEDs along its length, where a printhead has 300 pixels per inch (118 pixels per centimeter). LED printheads have been designed with as many as 600 pixels per inch (236 pixels per centimeter). In an exemplary embodiment it has been necessary to cut the length of such a die to plus or minus two micrometers and the width is cut to plus or minus five micrometers.
In addition to precise tolerances for cutting the dice, there are practical problems in arranging these LED bearing dice in a straight line with the necessary precision for good image quality. Also, spacing of the dice along the line is important for maintaining approximately the same spacing between LEDs at the ends of adjacent dice as there is between the LEDs on a die. Clearly economical as well as precise assembly techniques are important.
Further, once the LEDs are positioned, electrical contact must be made to each LED for application of current to cause illumination by the LED. The individual dice are typically mounted on a metal substrate which forms a common electrical contact for the cathodes of all of the LEDs on the die. A metal line for each LED is deposited on the front face of the die to carry current to the LED anode. Each line extends to an enlarged pad to which a metal wire is bonded. The wires lead to nearby integrated circuit chips which provide power for the LEDs and quite often additional functions for the printhead. Thus, in a typical printhead thousands of wire bonds may be required.
Thus, in addition to alleviating difficulties in precise positioning of the LEDs for a printhead, it would be desirable to reduce the number of wire bonds that need be made to the LED dice during assembly of an LED printhead.